This invention generally relates to the field of classifying finely divided particulate matter into fractions of different sized particles.
A wide variety of classification systems have been developed employing a classification zone wherein unclassified particles are projected into a moving fluid, such as air, to segregate the particulate matter into fractions according to their particulate size and/or density.
One of the more effective types of classification systems is that shown in U.S. Pat. No. 2,796,173 (Payne, et al.) and U.S. Pat. No. 3,334,74O (Hoffstrom). In these systems, particulate matter is fed to the top of a rapidly rotating table which accelerates the particulate matter to the rotational velocity of the table and projects the particles tangentially from the edge of the table into a horizontally and inwardly flowing fluid stream. Two forces act on the particles in the fluid stream, the centrifugal force which tends to push the particles outwardly and an aerodynamic drag force tending to sweep the particles inwardly with the fluid stream. The centrifugal force has a much more predominant effect on the larger or heavier particles than does the drag force so there is a good size separation between the particles thrown outwardly and the particles carried inwardly by the fluid stream.
The apparatus in the above separation systems were very complex and thus very expensive to manufacture and maintain. Moreover, once the desired cut point size between the various sized particle fractions was set, it could not be readily changed. The velocity of the gas through the separation zone could be varied to change the cutpoint size, but changing the velocity severely restricts the useful capacity of the separation system, particularly in a closed loop grinding system.
Another separation system, designed to conduct particle size analysis on small test samples of particulate matter, is shown in U.S. Pat. No. 3,141,337 (Hoffstrom). In this system, the particulate matter is projected radially through a fluid stream passing axially through an annular, classification zone and is collected according to the size and/or density of the particles on the inside of a removable wall which forms the outer boundary of the classification zone. This device was designed for particle analysis of small samples and not for the continuous classification of large quantities of particulate matter in an industrial facility.
Thus, there remains a need for a separation or classification system which can provide a sharp size separation of particles into a plurality of fractions and, particularly, one in which the cut off size between different sized fractions can be readily varied with a constant throughput capacity. The classification system of the present invention satisfies this need.